Particle display with jet-printed color filters and surface coatings

ABSTRACT

A method of forming a buried aperture in a nitride light emitting device is described. The method involves forming an aperture layer, typically an amorphous or polycrystalline material over an active layer that includes a nitride material. The aperture layer material typically also includes nitride. The aperture layer is etched to create an aperture which is then filled with a conducting material by epitaxial regrowth. The amorphous layer is crystallized forming an electrically resistive material during or before regrowth. The conducting aperture in the electrically resistive material is well suited for directing current into a light emitting region of the active layer.

BACKGROUND

GaAs and InP optoelectronic devices, including vertical cavitysurface-emitting lasers (VCSELs) and high performance diodes, often usea more electrically conducting “aperture” in a nonconducting layer todirect electrical current into a central light-emitting region or“active” region. In most of these GaAs and InP systems, aperturematerials are also selected to have a higher refractive index than thenonconducting layer material, thereby enabling the aperture to alsoconfine the generated optical fields. See U.S. Pat. No. 7,160,749entitled “Method and Structure for Eliminating Polarization Instabilityin Laterally Oxidized VCSELs” by Chua et al. which is herebyincorporated by reference in its entirety. In some “anti-resonant”structures, the refractive index of the aperture is designed to have alower value than surrounding areas to controllably induce losses tohigher order modes.

Various methods are available for forming the conducting aperture. Inone method, a chemical etch to form a pattern followed by a regrowth ofthe aperture material in the patterned layer openings is used to formthe aperture. Such a procedure is described in D. Zhou and L. J. Mawst,Appl. Phys. Lett., v.76 (13), 2000, pp. 1659-1661_which is herebyincorporated by reference. Alternate methods of forming a conductingaperture in an AlGaAs layer include converting select non-apertureregions of the AlGaAs layer into an insulating oxide through selectivewet thermal oxidation. The oxidized material also has a lower refractiveindex than the unoxidized material.

Although conducting apertures and/or light guiding apertures wouldbenefit an indium aluminum gallium nitride (InAlGaN) light emittingdevice, a suitable means for forming such apertures has not beenavailable. Chemical etching of nitride heterostructures followed bysubsequent regrowth in patterned layer openings has proved difficultbecause InAlGaN crystalline alloys are very stable and are thereforeresistant to chemical attack. Oxidation techniques have also beendifficult to implement because nitride based materials are not easilyoxidized.

Thus a method of forming apertures that channel current and/or confineoptically generated fields in a nitride based light emitting structureis needed.

SUMMARY

A method of forming a current directing aperture in a nitrideoptoelectronic device is described. In the method, an aperture layerincluding nitride is deposited over a crystalline active layer that alsoincludes nitride. The deposition typically forms an amorphous orpolycrystalline aperture layer. The deposition of the aperture layeroccurs at a low temperature below 800 degrees centigrade. Apertures areetched into the aperture layer. Subsequently, a crystallineheterostructure layer that also includes nitride is regrown over theaperture layer, the crystalline heterostructure layer to produce aburied aperture that can be used to direct light or current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a VCSEL array incorporating buried apertures.

FIG. 2 shows an intermediate structure used in forming a light emittingoptoelectronic device, the intermediate structure including a lowtemperature amorphous aperture layer deposited over a nitride activelayer.

FIG. 3 shows a side cross sectional view of an intermediate structurethat used in fabricating a VCSEL, the intermediate structure includingan aperture etched into an aperture layer.

FIG. 4 shows a top view of one embodiment of an aperture layer.

FIG. 5 shows a side cross sectional view of a crystallineheterostructure that has been regrown over a patterned aperture layer.

FIG. 6 shows a side cross sectional view of one embodiment of a VCSELwith a buried aperture.

FIG. 7 shows a top view of a nitride VCSEL with a buried aperture.

DETAILED DESCRIPTION

A structure including conducting apertures to channel current and/orconfine optically generated fields in a nitride light emitting deviceand a method of forming the structure is described. The method involvesforming and subsequently etching openings into a thin amorphous InAlGaNlayer, after which the InAlGaN layer is crystallized.

FIG. 1 shows a VCSEL array incorporating buried apertures 106 in anaperture layer 104. Electrodes 108 provide current 112 that flow throughtop mirror layer 116. Apertures 106 direct current into active regionsof active region layer 120. The current generates light in the activeregion. Top mirror layer 116 and bottom mirror layer 124 together form alaser cavity. Typically, the top mirror layer and the bottom mirrorlayer are distributed Bragg reflectors (DBRs) that confine light in theactive region to build up stimulated emissions. Emitted light 128 isoutput through apertures 106. As used herein, the term “apertures”refers to an “opening” that may or may not be filled with a material.However, when filled the characteristics of the aperture material issuch that the electrical conductivity is lower than the surroundingmaterial and/or the optical transmissivity is lower than the surroundingmaterial.

In a traditional GaAs optoelectronic light outputting device, theaperture may be formed by selective wet thermal oxidation of an AlGaAsaperture layer similar to the aperture layer 104. In one fabricationtechnique, an oxidizing agent enters gaps 132 between adjacent VCSELs inthe VCSEL array. The oxidizing agent gradually oxidizes aperture layer104 from the gap 132 perimeter towards the areas that will formapertures 106. Oxidation is terminated before the region to formapertures 106 is oxidized. When aperture layer 104 is AlGaAs, anoxidized AlGaAs bordering an unoxidized AlGaAs aperture results. Theaperture's unoxidized AlGaAs has a higher refractive index then thesurrounding oxidized AlGaAs thereby providing optical guiding of emittedlight. The oxidized AlGaAs also has a lower electrical resistivity thanthe surrounding AlGaAs thereby directing current through the apertureand the device active region. The described procedure for forming anaperture in an AlGaAs layer is described in U.S. Pat. No. 7,160,749 byChua et al. which is hereby incorporated by reference.

In the present invention, the VCSEL structure shown in FIG. 1 as well asanalogous light emitting structures may be fabricated with an InAlGaNaperture layer. In such a device, the InAlGaN aperture layer correspondsto aperture layer 104. FIGS. 2-5 show examples of the steps used to formsuch a structure.

FIG. 2 shows an AlGaN DBR mirror layer 204 formed over a sapphireheterostructure substrate 208. DBR mirror layers are commonly employedin resonant cavity devices such as lasers. AlGaN DBR mirrors aretypically fabricated using alternating pairs ofAl_(p)Ga_(1-p)N/Al_(q)Ga_(1-q)N layers. Mirror layer growth iscontrolled such that alternating tensile and compressive strains achievea high index of refraction contrast between alternating layers toproduce a highly reflective structure at the wavelength of lightproduced by the active region. Such mirror formation is described invarious references including U.S. Pat. No. 6,775,314 which is herebyincorporated by reference.

An InAlGaN active layer 212 is grown, usually epitaxially using MOCVD,over mirror layer 204. The amount of In and Al relative to Ga is chosenbased on the design of the specific structure and on the desiredemission wavelength. Such growth techniques are described in U.S. Pat.No. 6,285,696 by Bour et al entitled “AlGAInN Pendeoepitaxy LED andLaser Diode Structures for Pure Blue or Green Emission” which is herebyincorporated by reference. After active layer 212 formation, anamorphous aperture layer 216 is deposited at low temperatures, typicallyaround 550 degrees Centigrade, (although the temperatures typicallyrange from 400 to 800 degrees centigrade) over active layer 212. A lowerapproximate limit of 400 degrees occurs because below that temperature,it becomes difficult to crack ammonia which is often used as a nitrogensource for nitride films. However, using a catalyst or other methods tocrack ammonia at a lower temperature, or using a different source ofnitrogen altogether can enable deposition of nitride films at much lowertemperatures, including room temperature. Above approximately 800degrees centigrade, the amorphous material usually crystallizes.Typically the amorphous aperture layer is AlN, although GaN and otheralloys of In_(a)Al_(b)Ga_(1-a-b)N may also be used. The aperture layermaterial is preferably a nitride material that may be deposited at lowtemperatures in amorphous form and subsequently crystallized over theactive layer 212.

Amorphous aperture layer 216 is usually thin, typically between 5 and100 nm thick with an example thickness on the order of 20 nm. Keepingthe amorphous aperture layer thin prevents cracking during subsequentgrowths. The heterostructure underlying the amorphous aperture layer istypically a crystalline In_(x)Al_(y)Ga_(1-x-y)N layer grown attemperatures ranging from 700 degrees to 1200 degrees centigrade where xand y ranges between 0 and 1 depending on the desired devicecharacteristics.

After thin amorphous aperture layer 216 formation, the wafer is cooled,typically to a temperature below 100 degrees centigrade, more typicallyto room temperature and removed from the growth reactor for subsequentpatterning and etching. Standard photolithographic techniques may beused to pattern the aperture layer 216. In particular, the aperturelayer may be coated with a photoresist, masked and exposed to UVradiation that crosslinks unmasked regions of the photoresist. Theunmasked regions of the photoresist are removed creating openings in thephotoresist. The patterned openings allow an etchant to etchcorresponding apertures into the amorphous aperture layer.

Although most of the specification describes a MOCVD-grown amorphousaperture layer 216, in an alternate embodiment, the aperture layer canalso be deposited by physical vapor deposition (PVD). In the case ofPVD, the deposition temperature can be much lower, and the layer can bedeposited in polycrystalline form. As in the case of an amorphousaperture layer, and unlike a single crystal aperture layer, thepolycrystalline aperture layer can be patterned and etched as describedto form the apertures followed by a subsequent regrowth of material. Asused herein, “single crystal” aperture layer means an aperture layerwhere the crystal lattice order of the aperture layer is unbroken overan extended area. “Non-single crystalline material” as used herein isbroadly defined to include any material that is not a single crystal,including amorphous materials and poly-crystalline materials.

FIG. 3 shows a side view and FIG. 4 shows a top view of an exampleaperture 304 formed in amorphous aperture layer 308. Although FIG. 4shows a circular aperture opening, the aperture opening may be a varietyof shapes including but not limited to rectangular and triangularshapes. The aperture shape and area can define the active region andlight guiding properties of the aperture, thus the aperture maydetermine the beam profile output from the optoelectronic device.Typically, aperture areas range between 4 and 40,000 μm².

Various method may be used to form or etch the aperture opening. Onemethod of forming the aperture opening uses chemicals such as phosphoricacid to wet etch an amorphous AlN aperture layer. Because the chemicalsdo not attack crystalline nitride structures, the underlying crystallinenitride layer 304 forms a natural etch stop. In one embodiment, thecrystalline nitride layer 312 that forms the etch stop is the activelayer (light emitting layer) of the optoelectronic device (such asactive layer 212). Although wet chemical etching offers improvedselectivity and reduced contamination potential, dry etching techniquesmay also be used to create aperture 304.

After etching, FIG. 5 shows additional InAlGaN heterostructure layers504 regrown on the patterned surface. Regrowth typically occurs in aMOCVD reactor. The high temperatures, typically between 700 and 1200degrees centigrade, used in regrowing InAlGaN layers 504 is typicallysufficient to automatically crystallize an AlN amorphous aperture layer.Although crystallizing the amorphous aperture layer is not necessarycrystallizing the amorphous aperture layer after patterning ensuresgrowth of high quality films throughout the wafer area, not just at theaperture region, during subsequent growths. Crystallization of theamorphous aperture layer can typically be accomplished in-situ duringthe regrowth process. However, when the temperature is insufficient tocrystallize amorphous aperture layer 308 materials, an anneal step witha temperature of over a thousand degrees (typically around 1025 degreescentigrade) may be included in the MOCVD growth recipe prior to flowingmetal organics. A similar technique for growing a high quality nitrideheterostructure on a sapphire substrate using low temperature AlNnucleation layers that are annealed prior to growing additional InAlGaNepitaxial layers is described in U.S. Pat. No. 6,537,513 entitled“Semiconductor Substrate and Method for Making the Same” by Amano et al.which is hereby incorporated by reference in its entirety.

Regrowth layers 504 which include the material filling aperture region516 are typically epitaxially grown and doped p-type to make the regrownlayers electrically conductive. The now crystalline material of aperturematerial layer 308 (typically AlN) surrounding the aperture iselectrically resistive. Thus filled aperture region 516 serves as anaperture that funnels injected current into an active region in activelayer 312. Because in one embodiment, AlN has a higher refractive indexthan the surrounding InAlGaN material filling aperture region 516, theaperture also guides and confines generated light within the apertureregion. In one embodiment, the regrown InAlGaN heterostructure layer 504also serves as a current spreading layer that spreads current from acurrent source, typically metal contacts formed over heterostructurelayer 504.

FIG. 6 shows a cross sectional view and FIG. 7 shows a top view of oneembodiment of using the structure of FIG. 5 in a resonant cavitylight-emitting device. In FIG. 6, a metal contact 604 is formed overcurrent spreading layer 504. In the illustrated embodiment, metalcontact 604 is an annular p-metal contact that provides electricalcurrent 612. Aperture region 516 channels current 612 into the nitridecontaining active regions of active layer 212 where electrons and holesrecombine to generate emitted photons. The current travels throughmirror layer 316 to an n contact. In the embodiment shown in FIG. 6, thep and n contacts are formed on the same side of the wafer. This may bedone by etching mesas and forming one electrode, typically the p-contact604 in an upper portion of the mesa and another electrode, typically then-contact 620 laterally adjacent to a bottom portion of the mesa asshown. In an alternate embodiment, the n contact may be formed betweenthe mirror layer 316 and the sapphire substrate 320 using a laser liftoff process as described in U.S. Pat. No. 6,455,340 entitled “Method OfFabricating GaN Semiconductor Structures Using Laser-Assisted EpitaxialLift-Off” by Chua et al. and hereby incorporated by reference.

In FIG. 6, a top set of mirrors 616 are formed over the currentspreading layer (also called regrowth layer) 504 and the metal contact604. In one embodiment, the top mirror set are alternating layers ofquarter wave thick distributed Bragg reflectors which may be formedusing dielectric materials deposited by electron beam evaporation. Anexample material for forming mirror layer 616 includes silicon oxide.Upper mirror layer 616 and lower mirror layer 204 together form aFabry-Perot cavity for a resonant cavity light-emitting diode. If thereflectivity of the mirrors is sufficiently high, the device can operateas a vertical-cavity surface-emitting laser (VCSEL). The cavity enhanceslight emission at the resonant mode.

FIG. 7 shows a top view of the example light emitting device. FIG. 7shows a p-metal routing wire 704 that connects a p metal contact 708 toa source of electrical current (not shown). An insulating passivationlayer such as SiO₂ or Si₃N₄ separates the wire from the wafer surface.

Current flows along the routing wire to the p metal contact. The currentspreading layer distributes electrical current laterally from theannular metal contact to the central aperture region in the aperturelayer. Each aperture, such as buried aperture 712, directs current intoan active region of the active layer where the current exits the devicevia a second contact. Dielectric DBR mirrors may be evaporated over alarge area to cover many devices at once. The final resulting devicesmay emit light from a top surface or from a bottom surface depending onthe intended application. Bottom emitting devices provide for relativelysimple fabrication because it is easier to form a highly reflectivee-beam evaporated top dielectric mirror than a highly reflective MOCVDgrown bottom epitaxial mirror. When sufficient low optical losses areachieved, the resonant cavity device performs as a vertical cavitysurface-emitting laser.

The preceding description includes a number of details that are providedto facilitate understanding of the invention, and should not beinterpreted to thus limit the invention. Instead, the scope of theinvention should be defined by the claims, as originally presented andas they may be amended, encompass variations, alternatives,modifications, improvements, equivalents, and substantial equivalents ofthe embodiments and teachings disclosed herein, including those that arepresently unforeseen or unappreciated, and that, for example, may arisefrom applicants/patentees and others.

1. A method of forming a current directing aperture in a nitrideoptoelectronic device comprising: fabricating a crystalline active layerto emit light, the active layer to include nitride; depositing anaperture layer over the active layer, the deposition occurring at atemperature below approximately 800 degrees centigrade such that theaperture layer does not initially form a single crystal layer; etchingthe aperture layer to form an aperture; and, regrowing a crystallineheterostructure layer that includes nitride over the aperture layer toproduce a buried aperture that directs current.
 2. The method of claim 1wherein the aperture layer deposited forms an amorphous aperture layer.3. The method of claim 2 wherein the regrowing of the crystallineheterostructure crystallizes the amorphous aperture layer.
 4. The methodof claim 2 further comprising the operation of annealing the amorphousaperture layer at a temperature in excess of 700 degrees centigrade suchthat the amorphous aperture layer forms a single crystal aperture layer.5. The method of claim 1 wherein the active layer serves as an etch stopduring the etching of the aperture layer.
 6. The method of claim 1further comprising the operation of doping the regrown crystallineheterostructure such that the conductivity of the crystallized amorphousaperture layer is lower than the conductivity of the regrown crystallineheterostructure material.
 7. The method of claim 1 wherein the aperturelayer is formed from aluminum nitride.
 8. The method of claim 1 whereinthe crystalline active layer is formed from an alloy selected from agroup consisting of indium, aluminum, gallium, nitrogen and thecrystalline heterostructure layer is also an alloy selected from a groupconsisting of indium, aluminum, gallium, nitrogen.
 9. The method ofclaim 1 wherein the deposited aperture layer forms a polycrystallineaperture layer.
 10. The method of claim 1 further comprising: forming acontact above the regrown crystalline heterostructure, the contact toprovide current such that the regrown crystalline heterostructure servesas a current spreading layer.
 11. The method of claim 1 furthercomprising: forming a set of mirrors above the crystalline active layerand a second set of mirrors below the crystalline active layer such thatthe nitride optoelectronic device forms a resonant-cavity light-emittingdevice.
 12. The method of claim 1 wherein the aperture layer isdeposited by physical vapor deposition.
 13. An optoelectronic deviceincluding a buried aperture comprising: an active region that emitslight in an active layer, the active region including a nitride; acrystalline electrically resistive aperture layer formed over the activeregion, the electrically resistive layer including an aperture; and, asecond crystalline layer that includes nitride formed over resistiveaperture layer, the second crystalline layer doped to be electricallyconductive and fill in the aperture in the electrically resistive layer,the second crystalline layer in combination with the electricallyresistive aperture layer to direct current into a select active regionin the active layer to generate light.
 14. The optoelectronic device ofclaim 13 wherein the electrically resistive aperture layer includes anitride material.
 15. The optoelectronic device of claim 13 furthercomprising a distributed bragg mirror structure formed on one side ofthe active region, a second distributed bragg mirror structure formed ona second side of the second crystalline layer such that theoptoelectronic device forms a resonant-cavity light-emitting device. 16.The optoelectronic device of claim 13 wherein the electrically resistivelayer is amorphous aluminum nitride crystallized by thermal anneal. 17.The optoelectronic device of claim 13 wherein the electrically resistivelayer is amorphous aluminum nitride crystallized during regrowth of thesecond crystalline layer.
 18. The optoelectronic device of claim 13wherein the active region and the second crystalline layer are bothformed from InAlGaN.
 19. The optoelectronic device of claim 13 furthercomprising an electrical contact formed over the second crystallinelayer, the electrical contact to provide a current, the aperture in theelectrically resistive layer directing the current to an active regionwhere light is emitted.
 20. The optoelectronic device of claim 13wherein the index of refraction of the electrically resistive layer islower than the index of refraction of the aperture
 21. Theoptoelectronic device of claim 13 wherein the resistive aperture layeris formed directly over the active layer.
 22. An optoelectronic deviceto emit light, the optoelectronic device comprising: a sapphiresubstrate; an active layer including a nitride material formed over thesapphire substrate, an electrically resistive material including anaperture, the resistive material formed from a crystallized amorphousmaterial; a conducting material filling the aperture such that currentis directed from the aperture into an active region in the active layerwhere the current causes light generation.